Reliability of Mycobacteria growth indicator tube for testing susceptibility of Mycobacterium tuberculosis to ethambutol and streptomycin

Abstract

The reliability of mycobacterial growth indicator tubes (MGIT) for testing susceptibility of Mycobacterium tuberculosis to ethambutol and streptomycin was evaluated by comparing MGIT results to those obtained by the radiometric BACTEC TB system and the method of proportion. The method of proportion was considered the reference method. To resolve discrepancies, all three testing methods were repeated. For the 74 isolates evaluated, initial ethambutol results agreed by all three methods for 64 (86.5%) of them; 58 were susceptible and 6 were resistant. MGIT and method-of-proportion results agreed for 67 isolates, and BACTEC results agreed with method-of-proportion results for 71 (P = 0.096). Initial streptomycin results obtained by all three methods agreed for 69 (93.2%) isolates: 55 were susceptible and 14 were resistant. MGIT and method-of-proportion results were concordant for 69 isolates, and BACTEC and method-of-proportion results agreed for 73 (P = 0.049). The mean times to MGIT results were 5.58 ± 0.10 days (range, 3 to 9 days) for ethambutol and 5.47 ± 0.11 days (range, 3 to 9 days) for streptomycin, compared to a mean of 7.41 ± 0.20 days (range, 4 to 12 days) for both drugs with the BACTEC system (P < 0.001).